THIS invention relates to a method of transmitting data between stations in a cellular wireless communication system, and to a system which employs the method.
Conventional cellular wireless communication systems comprise a plurality of base stations each of which defines a cell of coverage. Mobile stations can communicate with a base station provided that they are within the relevant cell. In order to provide complete coverage of a predetermined geographical area, the location of base stations is selected so that the cells overlap, to minimize or eliminate xe2x80x9cdeadxe2x80x9d areas.
The above arrangement has the effect that each cell has a central, interference free zone, and an outer zone which overlaps with outer zones of one or more adjacent cells, in which interference can take place. In these latter areas, a mobile station transmitting to one base station will produce interference at the base stations of adjacent cells. In addition, in the overlapping zones, transmissions from adjacent base stations to mobile stations will interfere with each other.
This interference results in a reduction in capacity since base stations have to share resources (time slots, frequency channels or spreading code, for example) between themselves in communicating with other stations in the outer, overlapping zones.
In order to deal with this problem, conventional cellular networks are arranged so that adjacent base stations transmit using different resources (e.g. different frequencies and/or different time slots) to avoid interference. In a multi-cell network, this requires a high level of time synchronisation between the various base stations.
Since given resources cannot be used by adjacent base stations simultaneously, there is a reduction in the capacity provided by these base stations measured in erlangs/cell/Hz, for example. In addition, stations transmitting from outer zones have to use higher power to reach the base stations, which tends to result in reduced mobile station battery life and more expensive mobile stations.
According to the invention there is provided a method of transmitting data between stations in a cellular wireless communication system comprising a plurality of mobile stations and a plurality of base stations, the method comprising locating a plurality of base stations so that each base station has a zone of effective coverage which does not overlap with the zones of effective coverage of adjacent base stations, thereby defining zones of reduced coverage between the base stations, and relaying a data message from a sender station which is outside the zone of effective coverage of a destination base station to said destination base station via at least one mobile station.
The destination station may be a base station and the sender station may be a mobile station, in which case said at least one relay station may be located within the zone of effective coverage of the destination base station.
The data message from the sender station may instead or in addition be relayed by at least one relay station located in a zone of reduced coverage with respect to said destination base station.
The destination station, the sender station and said at least one relay station may all be mobile stations.
The zones of reduced coverage with respect to each base station are preferably zones in which at least one resource utilized by the base station is reduced.
The resources may include transmission power, transmission time slots, frequency channels, modulation efficiency and codes.
The resources are preferably reduced due to sharing thereof between two or more base stations in respective overlapping zones of reduced coverage of the base stations.
The relay stations may adjust their transmission power, when relaying messages to a base station or a mobile station in the zone of effective coverage of a base station, to avoid interference with said base station.
The relay stations may further adjust their usage of at least one of their transmission time slots, frequency channels, modulation efficiency and codes to avoid interference with said base station.
The relay stations may monitor data transmissions to and from base stations and/or between other mobile stations, and relay messages opportunistically when said data transmissions are not occurring, thereby sharing resources with other stations.
The relay stations may monitor data transmissions to and from base stations and/or between other mobile stations, and relay messages at a sufficiently low power level to avoid interference with said data transmissions.
The relay stations may monitor data transmissions to and from base stations and/or between other mobile stations and relay messages opportunistically, utilising higher efficiency modulation, when an increased signal to interference ratio is available, to avoid interference with said data transmissions.
Further according to the invention there is provided a cellular wireless communication system comprising a plurality of mobile stations and a plurality of base stations, each station being able to transmit data to and receive data from other stations, the base stations being located so that each base station has a zone of effective coverage which does not overlap with the zones of effective coverage of adjacent base stations, thereby defining zones of reduced coverage between adjacent base stations, and the mobile stations being adapted to relay data messages received from a sender station which is outside the zone of effective coverage of a destination base station to said destination base station.
Preferably, the zones of reduced coverage with respect to each base station are zones in which at least one resource utilized by each base station is reduced.
The resources may be, for example, transmission power, transmission time slots, frequency channels, modulation efficiency, or codes.
The resources may be reduced due to sharing thereof between two or more base stations in respective overlapping zones of reduced coverage of the base stations.
The mobile stations are preferably adapted to adjust their transmission power, when relaying messages to a base station or a mobile station in the zone of effective coverage of a base station, to avoid interference with said base station.
The mobile stations may further be adapted to adjust their usage of at least one of their transmission time slots, frequency channels, modulation efficiency and codes to avoid interference with said base station.
The mobile stations are preferably adapted to monitor data transmissions to and from base stations and/or between other mobile stations, and to relay messages opportunistically when said data transmissions are not occurring, or at a sufficiently low power level to avoid interference therewith, thereby to share resources with other stations.
The mobile stations may be adapted to monitor data transmissions to and from base stations and/or between other mobile stations and to relay messages opportunistically, utilising higher efficiency modulation, when an increased signal to interference ratio is available, to avoid interference with said data transmissions.